Is NetworkManager enabled/running on the test system? If so, please try again with NetworkManager disabled.
Is there actually a 90 second delay before the command fails? (as implied by the error message)
Also, is there any output in /var/log/messages during this time?

NetworkManager and bridges still do not play together well, and the documentation has always recommended disabling NetworkManager before adding a bridge. In RHEL6.3/6.4 the situation is better, but still not ideal, and it's still recommended that NetworkManager be disabled when using bridges.
If there is a particular place in the documentation that you think should specifically mention this, please file a bug against the documentation.
As for the maximum wait time of 90 seconds, this is because of the particular configuration of your ethernet device and the services that are using it; since a delay time on a bridge translates into having no network traffic on the interface for that number of seconds, if you have dhcp configured for the interface, but a delay longer than the timeout period of the dhcp client, you are doomed to failure. In other situations a longer delay could be perfectly valid, so we can't just set an arbitrary upper limit. Knowing that sort of detail for every protocol using the interface and then automatically limiting the stp delay based on those details is beyond the scope of this command. It may bear mentioning in the documentation somewhere, though. Again, if/when there is a specific piece of documentation for this command, please file a bug against the docs.
Since operation with NetworkManager enabled is not supported (and there is basically nothing libvirt can do about the issues that exist), and the other things mentioned are things that should be documented as limitations, I'm closing this bug. If you can find a specific docs component that should mention these limitations (maybe one of the virt guides has a section), you can either reopen this bug assigned to the doc component, or open a new bug.

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